A class of polyesters having certain advantages over polyethylene terephthalate is disclosed in German OS No. 2,520,819 (Schaefgen). These polyesters exhibit optical anisotropy in the molten state and can be melt spun into fibers which on heat treatment by techniques disclosed in German OS No. 2,520,820 (Luise) increase in tenacity. Such heat treated polyester fibers are suggested for use in tire cords and in other industrial and consumer products where the high strength permits use of less yarn with its attendant economic and other advantages. In such end-use applications, particularly in truck tires, exceptionally high temperatures, e.g. in excess of 150.degree. C., may be encountered. Therefore it is important that the yarn of the cord be of high strength at room temperature and also possess high strength at such elevated temperatures. Yarns from polyesters of the present invention satisfy this requirement.